This invention relates to an acoustic diaphragm used for a loudspeaker or the like and a method for producing such diaphragm. More particularly, it relates to an acoustic diaphragm employing a micro-fibrillated cellulose and a method for producing such diaphragm.
Up to now, cone paper made from pulp has been used extensively for an acoustic diaphragm for a loudspeaker or the like.
The cone paper is fabricated through the process steps of beating the pulp, dispersing and swelling the beaten pulp in water, and forming the pulp dispersed in water to the desired web shape by a process similar to a paper making process. However, the web obtained by simply dispersing the pulp obtained from wood in water by the process similar to the paper-making process can hardly be used as a diaphragm because it is destitute of a crisp feel and inferior in mechanical strength. The reason is that individual fibers making up the pulp are not affixed strongly together.
The affixing force may be developed by softening and disintegrating the fibers and into component fibrils (fibrillation) for increasing the number of contact points between the fibers for increasing the number of hydrogen bonds.
Such mechanical fibrillation of the individual fibers is termed beating and is usually performed by an apparatus known as a beater.
Meanwhile, a higher longitudinal wave propagating velocity or a higher sound propagating velocity C is required of the acoustic diaphragm, so that a material which is light and has a large Young's modulus may be advantageously employed as the diaphragm material.
The physical properties of the come paper, such as the Young's modulus or tensile strength, are determined by the degree of beating, as mentioned above, such that, in order to produce the cone paper exhibiting the higher values of the Young's modulus, it is necessary to employ a cellulose exhibiting the advanced degree of beating and hence of fibrillation. In other words, it is thought that, in the cone paper used as the diaphragm material, the higher the beating degree of the cellulose used for making the web, the higher becomes the Young's modulus of the cone paper.
However, if the cellulose used for making the web of the cone paper is beaten to a higher degree, the strength of the cellulose in the wet state during the web-making process is drastically lowered, so that difficulties are presented with respect to handling and shape retention. For example, if it is attempted to transfer the formed web in the wet state to another metallic mold, the web may be collapsed in shape.
On the other hand, the cellulose tends to be intruded into the meshes of the wire screen of a web-making apparatus, so that, when it is attempted to peel off the formed web (cone paper) from the wire screen after drying, an excess force tends to be applied momentarily to the web to destroy the web due to the higher rigidity of the wire screen of the web-forming apparatus.
On the other hand, when a flat web is formed and molded to a desired shape by press working with the aid of a metallic mold, an excess force must be used that tends to destroy the web.
Therefore, owing to production difficulties, it is thought to be difficult to make the web for the acoustic diaphragm from the cellulose which has been fibrillated to an excessively high extent even though such high degree of fibrillation is expected to be desirable from the viewpoint of characteristics. Above all, it is thought to be extremely difficult to produce the diaphragm with a reduced thickness.